The present invention relates to an apparatus for chemically etching a wafer-like material in an etching solution or, more particularly, to an apparatus in which a wafer-like material such as a semiconductor silicon wafer can be readily finished by a chemical etching means in an etching solution to be imparted with highly planiform or flat surfaces with good parallelism.
As is well known, semiconductor silicon wafers, from which various kinds of electronic devices are fabricated, are prepared by grinding and soiling a single crystal rod of high purity silicon into thin wafer-like forms followed by lapping and mirror-polishing of the surfaces. It is therefore unavoidable that the surface layer of the thus polished silicon wafer is more or less under mechanical strain caused in the above mentioned steps of mechanical working. Accordingly, it is a usual practice that the strained surface layer of the silicon wafer is removed by chemically etching the surface in an etching solution before the silicon wafer is further processed into electronic devices.
It is of course essential in this chemical etching that the reaction with the etching solution proceeds uniformly over whole surface of the wafer by preventing localized oxidation reaction and the etching is terminated at one time so that the uniformity in the removed amount of the surface layer is ensured from portion to portion on the wafer surface resulting in highly planiform surfaces with excellent parallelism of the finished wafer material.
Such a uniform proceeding of the etching reaction is obtained when the oxidation product formed in the etching solution on the wafer surface by the reaction with the etching solution is rapidly diffused out of the surface region together with dissipation of the heat of reaction to be replaced with a fresh portion of the etching solution and the etching solution is brought into contact with the wafer surface in a turbulent flow so that the means contacting distance of a volume fraction of the etching solution with the wafer surface is minimized.
In order to ensure the above mentioned desirable conditions of contacting between the wafer surface and the etching solution, various types of apparatuses have been proposed in the prior art to be used for the chemical etching of wafer materials. In an example, a plural number of the wafer materials are held horizontally or vertically in a casing in parallel with each other with narrow and uniform spaces therebetween and the flow of the etching solution in the spaces between the parallel wafers is accelerated by rotating each of the wafers around its own axis or around an outer axis or by utilizing the turbulent effect of the rising bubbles.
The above mentioned prior art apparatuses are, however, not always quite satisfactory in respect of the uniformity of the etching reaction due to the localized reaction of the etching solution staying between the oppositely facing wafers leading to the accumulation of the reaction product as well as to the localized temperature elevation of the etching solution. The difficulty due to the above mentioned problem of the non-uniform oxidation reaction is increased more and more with the increase in the diameter of the semiconductor wafers as a trend of the semiconductor technology in recent years. In this regard, no satisfactory apparatus for chemical etching of a wafer material in an etching solution is available hitherto.